This invention relates to the unexpected discovery of a class of surfactants which demonstrate improved surfactant recovery from aqueous solutions upon ultrafiltration.
Aqueous waste water solutions from industrial metal cleaning processes such as metal stamping, grinding, oil drilling and machining operations contain hydrocarbon and inorganic waste materials as well as the surfactants used in the cleaning process. Because of environmental and economic concerns, it is important to remove and desirably recover for reuse the active surfactant ingredients from these aqueous by-product or waste solutions. The recovery and recycling of surfactants from treated aqueous solutions not only reduces or eliminates the discharge of contaminated water into the environment but also reduces the overall cost of industrial processes. The separation and recovery of these surfactants from the waste water is of particular importance in processing aqueous solutions containing surfactants as they can be a costly component of the solution.
Ultrafiltration equipment can be used to overcome certain waste disposal problems in connection with aqueous solutions, whether using polymeric or sintered metal/sintered metal oxide filters. The economics of these processes in recovering surfactants have been seriously hampered since many of the conventional surfactants are not recoverable in the permeate. The ability to recycle and reuse these surfactants from aqueous solutions is significantly reduced thereby necessitating the addition of more surfactant to bring the amount in solution to the original level. This is a significant problem in the treatment of contaminated water systems since the loss of the original surfactant and the subsequent need for the addition of more becomes expensive.
Waste water treatment systems have been disclosed that claim significant recovery of water-based cleaning systems in the permeate upon ultrafiltration although in reality considerable dissatisfaction has been expressed concerning the filterability and recovery of existing water-based cleaning formulations. Whereas U.S. Pat. No. 5,205,937 to Bhave et. al. asserts to disclose aqueous cleaning systems wherein high percentages of the cleaner passes through the filter in the permeate for recycling, the amount of cleaner in the permeate is measured by HCI titration which is actually a measure of the caustic or sodium hydroxide present, not the surfactant. Most of the nonionic surfactants disclosed therein do not in fact pass through the membrane.
It is known that certain sultaine surfactants as disclosed in U.S. Pat. No. 4,891,159 to Nadolsky provide effective surfactant properties in highly alkaline cleaning solutions making them useful in paint stripping compositions, hard surface cleaners, oven cleaners, wax strippers, degreasers, aluminum cleaners, and bottle washing compositions. At lower alkalinities, they have proven effective in laundry and dish washing detergents and hand cleansers. However, these surfactants are not taught to provide any significant benefits when used in connection with an ultrafiltration-based water treatment and surfactant separation and recovery system.
As discussed above, U.S. Pat. No. 5,205,937 to Bhave et al. discloses a method for the recovery of a surfactant-containing solution from an aqueous contaminant stream using nonionic surfactants, among others. Whereas high yields of cleaning formulation is asserted to be recovered from the permeate, the amount that comprises the surfactant is really unknown since only the amount of sodium hydroxide present in the system is determined through the titration of the permeate with hydrochloric acid.
U.S. Pat. No. 5,654,480 to Dahanayake et al. on the other hand discloses the use of certain amphoteric surfactants, namely sultaines, in aqueous contaminant-containing streams. These surfactants, due to the nature of their pH dependent charge, are able to pass through the ultra-filtration membrane and are recovered in the permeate in very high amounts.
The present invention is directed to the finding that surfactant-containing aqueous compositions containing a certain class of nonionic surfactants unexpectedly provide improved surfactant recovery in the permeate after ultrafiltration. More particularly, it is discovered that a certain class of alcohol alkoxylate surfactants provide superior surface activity in aqueous cleaner systems while at the same time are able to pass through the filtration membranes for efficient recycling thereof.